Wow this brings back a lot of memories.
I must have made a couple hundred pounds of this stuff from the same recipe while my kids were growing up. The last batch I made was probably 10-12 years ago. The trick is not to overcook it. Just like gravy or any other flour-based liquid that you heat up over the stove, when it gets to a certain temperature, it suddenly thickens up. That's when you pull it from the heat to stop it from cooking. My son's loved playing with the stuff and they got a nearly unlimited supply. We went through a lot of flour, salt and cream of tartar.